DIVERGENT EVOLUTION THROUGH SEGREGATION. | 309 
Tn order that impregnational segregation should be established and 
perpetuated it is necessary: First, that variation should arise from 
which it results that those of one kind are capable of producing vigor- 
ous and fertile offspring in greater numbers when breeding with each 
other than when breeding with other kinds; second, that mutually 
compatible forms should be so brought together as to insure propaga- 
tion through a series of generations. In order to secure this second 
condition it is necessary that, in the case of plants, there should be 
some degree of local, germinal, or floral segregation, and in the case of 
animals that pair, either pronounced local segregation, or partial local 
segregation supplemented by social or sexual segregation. The first 
of these factors I call negative segregation, as contrasted with all 
other forms of segregation, which I group together as positive segre- 
gation. 
Of each form of segregation which we have up to this point consid- 
ered, the segregating cause has been one that distributes individuals 
of the same species in groups between which free inter-generation is 
checked; while the propagation of the different groups depends simply 
on the original capacity for inter-generating common to all the members 
of the species. The inter-crossing has been limited not by the capacity 
but by the opportunity and inclination of the members. Coming now 
to cases in which the lack of capacity is the cause that checks the pro- 
duction of mongrels, we find a dependence of a very different kind; for 
to insure the propagation of the different groups it is not enough that 
the general opportunity for the members to meet and consort remains 
unimpaired. There must be some additional segregating influence 
bringing the members together in groups corresponding to their seg- 
regate capacity, or they will fail of being propagated. 
A partial exception must be made in the case of potential and 
pre-potential segregation, the latter being due to the pre-potency of 
the pollen of a species or variety, on the stigma of the same species 
or variety and the former to the complete impotence of the foreign 
pollen. When allied species of plants are promiscuously distrib- 
uted over the same districts, and flowering at the same time, pre-po- 
tency of this kind is one of the most direct and efficient causes of 
segregate breeding. The same must be true of varieties similarly dis- 
tributed whenever this character begins to affect them. In the case, 
however, of dicecious plants and of plants whose ovules are incapable of 
being impregnated by pollen from the same plant, no single plant can 
propagate the species. If therefore the individuals so varying as to 
be pre-potent with each other are very few and are evenly distributed 
amongst a vast number of the original form, they will fail of being 
segregated through failing to receive any of the pre-potent pollen. It 
is thus apparent that when the mutually pre-potent form is represented 
by comparatively few individuals, their propagation without crossing 
will depend on their being self-fertile and subject to germinal or floral 
